1. Field of the Invention
The present invention relates to a fault detecting apparatus for a four-wheel drive vehicle which transmits a driving force of a motive force directly to a left and a right main driving wheel and to a left and a right sub-driving wheel through a clutch, and is equipped with a speed increasing system for increasing rotational speeds of the sub-driving wheels above rotational speeds of the main driving wheels.
2. Description of the Prior Art
A conventional four-wheel drive vehicle of the type mentioned above is known, for example, from one disclosed in Japanese Patent Publication No. 7-64219. This four-wheel drive vehicle (hereinafter simply called the “vehicle”) belongs to the FF type which has an engine arranged, for example, in a front region to use front wheels as main driving wheels and rear wheels as sub-driving wheels. A speed increasing system for transmitting the rotation of a propeller shaft to the rear wheels at an increased speed, and a left and a right hydraulic clutch for distributing a transmitted torque to the left and right rear wheels are disposed between the propeller shaft and the left and right rear wheels.
The speed increasing system comprises an input shaft connected to the propeller shaft, a direct-coupled clutch connected to the input shaft, a speed increasing clutch, an output shaft connected to the rear wheels, a planetary gear mechanism disposed between the direct-coupled clutch and the output shaft. The direct-coupled clutch and the speed increasing clutch are both of hydraulic type, and are controlled for connection and breakage by a piston which moves in accordance with oil pressures supplied to respective oil pressure chambers associated therewith. When the direct-coupled clutch is connected while the speed increasing clutch is broken, the input shaft is directly coupled to the output shaft through the direct-coupled clutch, thereby permitting the output shaft to rotate at the same rotational speed as the input shaft. On the other hand, when the direct-coupled clutch is broken while the speed increasing clutch is connected, the rotation of the input shaft is transmitted to the output shaft through the planetary gear mechanism. Since a total gear change ratio of the planetary gear mechanism is set to a value smaller than 1.0, the rotation of the input shaft is increased before it is transmitted to the output shaft.
Fastening forces of the left and right hydraulic clutches are variably controlled independently of each other by the piston which moves in accordance with oil pressures supplied to the respective oil pressure chambers associated therewith. With the foregoing configuration, the rotational speeds of the rear wheels, which are sub-driving wheels, can be increased above the rotational speeds of the front wheels, which are main driving wheels, by connecting the speed increasing clutch and increasing the fastening forces of the respective hydraulic clutches. As such, a stable cornering ability can be ensured, for example, by controlling the vehicle, during cornering, to increase the rotational speed of the rear wheel on a cornering side above an average rotational speed of the front wheels.
In the vehicle as described above, a fault can be detected in the speed increasing system based, for example, on a relation between the rotational speeds of the input shaft and the output shaft. However, when the speed increasing system is switched from the direct-coupled state to the speed increasing state, and is switched from the speed increasing sate to the direct-coupled state, a response delay of the speed increasing system causes a certain delay until the speed increasing system is actually switched to an intended operating state after an instruction signal is generated to instruct the switching, possibly resulting in an erroneous detection when the aforementioned approach is based to detect a fault. Particularly, the conventional speed increasing system described above is highly susceptible to erroneous detections due to a low responsibility of hydraulic pistons associated with the direct-coupled clutch and the speed increasing clutch, both of which comprise hydraulic clutches. Also, in a situation in which the rotational speed of the front wheels or the rear wheels suddenly fluctuates, the fluctuations result in fluctuations in the relationship between the rotational speeds of the input shaft and the output shaft connected to the front wheels and the rear wheels, respectively, so that a fault detection based on the foregoing approach would possibly result in erroneous detections.